Acoustic measurement system incorporating a temperature controlled waveguide

ABSTRACT

For a gas turbine engine ( 10 ), a method and apparatus for determining temperature of combustion gas in a flow path ( 17 ) of the engine. In one embodiment a waveguide ( 30 ) includes a cavity ( 36 ) with a first waveguide end ( 32 ) including a barrier layer ( 58 ) which may be a membrane. With the first end positioned to inject an acoustic signal into the flow path, the barrier layer ( 58 ) isolates the cavity from combustion gas. Acoustic instrumentation positioned along the surface ( 38   i ) of a wall ( 38 ) of the waveguide provides generation and propagation of acoustic signals into the flow path for detection and measurement after reflection back into the cavity. The wall may include sections of two different materials, one material having a higher melting temperature than the other, or one material having greater electrically isolating properties than the other section.

RELATED APPLICATION

This application is related to subject matter disclosed in U.S. patentapplication Ser. No. 14/800,763 filed Jul. 16, 2015.

FIELD OF THE INVENTION

The present invention relates to turbine engines and, more particularly,to turbine engines incorporating systems and methods for determiningcombustion gas temperature based on measurement of acoustic signals. Inone application such temperature measurement is used to controloperation of a gas turbine engine.

BACKGROUND OF THE INVENTION

Combustion turbines, such as gas turbine engines, generally comprise acompressor section, a combustor section, a turbine section and anexhaust section. In operation, the compressor section can induct andcompress ambient air. The combustor section generally may include aplurality of combustors for receiving the compressed air and mixing itwith fuel to form a fuel/air mixture. The fuel/air mixture is combustedby each of the combustors to form a hot working gas that may be routedto the turbine section where it is expanded through alternating rows ofstationary airfoils and rotating airfoils and used to generate powerthat can drive a rotor. The expanding gas exiting the turbine sectioncan be exhausted from the engine via the exhaust section.

The fuel/air mixture at the individual combustors is controlled duringoperation of the engine to maintain one or more operatingcharacteristics within a predetermined range, such as, for example, tomaintain a desired efficiency and/or power output, control pollutantlevels, prevent pressure oscillations and prevent flameouts. In a knowntype of control arrangement, a bulk turbine exhaust temperature may alsobe monitored as a parameter indicative of a condition in the combustorsection. For example, a controller may monitor a measured turbineexhaust temperature relative to a reference temperature value, and ameasured change in temperature may result in the controller changing thefuel/air ratio at the combustor section.

In a known temperature monitoring system for controlling combustionoperations, temperature monitors, such as thermocouples, are locateddirectly in the exhaust flow of the turbine. Such monitoring systemshave generally required locating thermocouples at different fixed axiallocations along the exhaust flow, which may introduce uncertainties inrelation to temperature calculations for controlling the engine asconditions affecting operation of the engine change, such as a varyingload condition on the engine.

BRIEF DESCRIPTION OF THE DRAWINGS

An understanding of the invention will be had by reading the followingdetailed description in conjunction with the accompanying drawings,wherein:

FIG. 1 is a perspective cross-sectional view of a gas turbine engineincorporating an acoustic temperature measurement system in accordancewith the present invention;

FIG. 2 illustrates features of an acoustic measurement system fordetermining operating temperature of a combustion gas based on time offlight for an acoustic signal passing through a combustion gas flowpath;

FIG. 3 illustrates features of an acoustic temperature measurementsystem according to another embodiment with a waveguide comprising firstand second sections comprising having different materials properties;and

FIG. 4 schematically illustrates a series of sequentially formed layersL_(i) along a portion of a transition region between the first andsecond sections of a waveguide wall shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

The invention is illustrated for an embodiment of a gas turbine engine10 comprising an acoustic temperature measurement system whichdetermines temperatures of exhaust combustion gas along a flow pathextending from the combustor section through the turbine section. Asdescribed in Patent Application Publication US 2015/0063411, (assignedto the assignee of the present invention, filed Sep. 4, 2013 andincorporated herein by reference) systems which measure the temperatureof a gas are useful for monitoring and control of engine operations.However, due to the high temperatures present in segments of the flowpath of the combustion gas, it has been difficult to provide accuratetemperature measurements on a long term.

More recently, indirect temperature measurements have been accomplishedwith acoustic methods to avoid placing temperature probes directlywithin the hot combustion gas flow. Because engine noise can hinderaccurate detection of sound signals transmitted for time of flightmeasurements, these systems have generated sound signals, distinct andrecognizable from sounds normally produced by the engine, to identifyacoustic measurement signals that have travelled through the gas flowpath. This imposes a high temperature constraint for the acousticsensors which can be addressed by injection of cooling gas to controlthe temperature of sensor operation. However, such cooling can affectthe time of flight measurements performed. Embodiments of the inventiondo not require flow of cooling fluid into the measurement region toprotect acoustic sensors.

Referring to FIG. 1, a portion of an exemplary gas turbine engine 10includes a compressor section 12, a combustor section 14, a turbinesection 18, and an exhaust section 20. The combustor section 14 includesa plurality of combustor baskets or combustors 16 and associatedtransition ducts 22, wherein the combustors 16 and transition ducts 22define a portion of a flow path 17 along an axial direction in which thecombustion gas flows to the exhaust section 20.

During operation of the engine 10, air output from the compressorsection 12 is provided to the combustor section 14 where it is combinedwith fuel in the combustors 16, and the fuel/air mixture is ignited toform the hot combustion gas which is expanded through the turbinesection 18 and then passed through the exhaust section 20.

Embodiments of the invention incorporate an acoustic temperaturemeasurement system 24 in the gas turbine engine 10 to determinecombustion gas temperatures and control operation of the engine 10 suchas, for example, by varying the air-to-fuel ratio. The partial view ofFIG. 2 illustrates a waveguide 30 in the acoustic temperaturemeasurement system 24 having a body in which there is formed a tapered,e.g., frustoconical, cavity 36 extending between first and secondwaveguide ends 32, 34. The wall 38 of the body of the waveguide, formedabout the cavity 36, has a cylindrically shaped exterior wall surface 38e and has, along an interior surface 38 i, a tapered shape complementaryto that of the tapered cavity 36.

In the example illustration the waveguide 30 adjoins a transition duct22 adjacent the combustor section 14 in the turbine 10. More generally,the gas turbine engine 10 may comprise a plurality of waveguides 30,each located at a position of interest along a wall 40 in the flow path17, including positions within the turbine section or along the wall ofa diffuser. The first waveguide end 32 is positioned within acylindrically shaped mounting sleeve 42 that extends outward from thewall 40. The second waveguide end 34 extends beyond the mounting sleeve42. As shown in FIG. 2, the mounting sleeve may have an inside diameter42 _(id) substantially larger (e.g., 0.5 to one mm) than the outsidediameter 38 _(od) of the cylindrically shaped exterior wall 38 of thewaveguide 30. This spaced arrangement provides a channel, C, between themounting sleeve 42 and the waveguide exterior wall surface 38 e.

A pair of acoustic transducer units, i.e., a transmitter unit 44 and areceiver unit 46, are mounted to the waveguide 30. In other embodiments,the waveguide may comprise one or more acoustic transceivers or multipletransmitters or multiple receivers. The transmitter unit 44 includes aninner portion 44 i and the receiver unit 46 includes an inner sensorportion 46 i that acoustically interfaces with the cavity 36 to emitacoustic signals into the cavity or to sense acoustic signals from thecavity. The transmitter unit 44 is mounted along the second waveguideend 34. The receiver unit 46 is attached to the mounting sleeve 42,e.g., approximately midway between the waveguide ends, and extendsthrough the sleeve, into the waveguide 30 to sense the acoustic signalsin the cavity 36 generated by the transmitter unit 44 and reflected backinto the waveguide.

The transmitter unit 44 and the receiver unit 46 are each connected to aprocessor or controller 54 that is configured to operate a signalgenerator 56 to produce predetermined output signals from thetransmitter and to receive time-of-flight signals, corresponding to theoutput signals, from the receiver unit 46. The controller 54 is furtherconfigured to store and process data corresponding to the receivedsignals to calculate temperatures and to produce outputs in accordancewith the calculated temperatures associated with the received signals,as described with detail in US Pub. 2015/0063411. The controller 54 mayalso be configured to provide signals for controlling operationsaffecting combustion, including sending of signals to the individualcombustors 16, including providing control of, for example, the fuel/airratio at the combustors 16.

To prevent ingress of combustion gas into the cavity 36 the waveguide 30is sealed from the flow path 17 by closing the first waveguide end witha membrane 58 capable of withstanding typical high temperatures of thecombustion gas, e.g., up to 1,200° F. (650 C). However, the firstwaveguide end 32 is also relatively transmissive to effect propagationand reflection of acoustic measurement signals from the transmitter unit44, through the first waveguide end 32, into a measurement region 60 inthe flow path 17, back through the first waveguide end 32 and into thewaveguide cavity 36 for sensing by the receiver unit 46. As indicatedwith an arrow 64, for an acoustic signal generated with the transmitterunit 44, a measurable portion of the transmitted signal is reflectedfrom a part of the wall 40 to return through the first waveguide end 32and into the cavity 36 for sensing by the receiver unit 46.

Consistent with the frustoconical shape of the cavity 36, a portionalong the waveguide first end 32 has an interior circular shape ofdiameter, D, and the membrane 58 at least spans the diameter, D, tocompletely cover and seal the first end 32. The membrane 58 may bebonded about the first end of the waveguide, e.g., by a weld or braze,to extend across an exterior region surrounding the waveguide end.Numerous other arrangements may effect the seal, including use of athreaded ring which matingly engages another threaded body to press andsecure the membrane against a surface adjoining the interior circularshape of diameter, D, along the first end 32, optionally with a gasketor sealing member there between. With this or another mechanicalcoupling arrangement the membrane 58 is removable for maintenance orreplacement. In other embodiments, the membrane may extend across themounting sleeve 42 by, for example, attachment across an opening nearthe interface 62 between the sleeve 42 and the wall 40 or by attachmentacross a wall opening. The membrane may be removable for maintenance orreplacement. Absent a hermetic seal between the membrane 58 and thecavity 36, when a cooling system sends high pressure air from theturbine compressor into the cavity 36, there may be slight leakage ofrelatively high pressure cooling air from the waveguide, past themembrane 58, and into relatively lower pressure combustion gas in theflow path 17.

The membrane may comprise titanium or a nickel alloy of sufficientthickness to assure integrity under turbine operating conditions whilealso permitting satisfactory acoustic transmission properties, e.g.,preferably less than 10 dB attenuation, to assure a satisfactory signalto noise ratio. In one embodiment the membrane comprises an alloy in theInconel 600 series (e.g., 601, 610 or 625) having a thickness in a rangeof 200-300 microns. In the illustrated embodiments the membrane 58 actsas a secondary diaphragm of an acoustic transmitter similar to anelectro-dynamic speaker. With the exemplary dimensions of the membraneand the exemplary attachment methods the membrane 58 can reproduce thesignal without unacceptable distortion relative to that originating fromthe transmitter unit 44.

The time-of-flight of the portion of the signal traveling between thetransmitter unit 44 and the receiver unit 46 is used to determine atemperature value for of the combustion gas travelling through theregion 60 at the time of measurement. The temperature determination isbased on the principle that the speed of sound in a gas changes as afunction of temperature. For a determined or known composition of thegas, it is possible to determine the temperature of the gas based on themeasured time for an acoustic signal to travel the distance between thetransmitter unit 44 and the receiver unit 46, i.e., based on the speedof the sound signal traveling through the gas.

The acoustic temperature measurement system 24 includes a coolingarrangement to control the temperature of the waveguide cavity 36, thewaveguide wall 38 and acoustic instrumentation such as the transmitterunit 44 and the receiver unit 46. A relatively cool fluid 70 is providedinto and around the waveguide 30 through an inlet port 74 which extendsinto the cavity 36. The fluid 70 exits the waveguide via an outlet port78 which also extends into the cavity 36. The inlet port 74 ispositioned close to the waveguide first end and the outlet port 78 maybe positioned approximately midway between the first and secondwaveguide ends causing the fluid 70 to travel in a direction away fromthe first end 32 and toward the second end 34. However, the direction offlow may be reversed. This arrangement predominantly sends the fluid 70through a portion of the cavity proximate the waveguide first end. Theinlet port may be in such close proximity to the membrane 58 that thecool fluid 70 impinges on the membrane. The inlet port 74 may alsoinclude or only include an opening to inject the fluid 70 into thechannel, C, and the outlet port 78 may also include an opening to allowthe cooling fluid 70 to exit the channel, C. This arrangement furtherfacilitates cooling exterior portions of the waveguide closest to theflow path 17. In still other designs the inlet and exit ports may onlysend the cooling fluid 70 directly into the channel, C, but thewaveguide wall 38 includes a series of ingress ports 82 near the firstwaveguide end through which the fluid 70 is sent into the cavity. Thewall also includes a series of egress ports 83 through which fluid 70 inthe cavity 36 returns into the channel, C. Multiple cooling paths havebeen illustrated for the embodiment shown in FIG. 2. Other embodimentsmay incorporate one of these apart from or in combination with one ormore other cooling path designs for the waveguide and acousticinstrumentation, e.g., sending cooling fluid 70 through the cavity 36only, or only providing cooling of the waveguide wall with the channelC, or providing multiple ones of the cooling arrangements such ascooling through both the channel C and the cavity by sending fluid 70through the ports 82, 83. In the past, waveguides suitable for acoustictemperature measurement systems have been cast or machined from steel inorder for the end portion of the waveguide adjoining the flow path towithstand high temperatures ranging, for example, up to 650 C. However,with the waveguide having a thermal conductivity typical of a metal, ithas been difficult to limit thermal conduction to prevent thermal energypresent in the adjoining flow path 17 from eventually elevating thetemperature in the vicinity of the acoustic instrumentation. This is aconcern even when the instrumentation is at the end 34 of the waveguidefarthest away from the flow path 17.

The partial view of FIG. 3 illustrates an acoustic temperaturemeasurement system 80 comprising a hybrid waveguide 84, the features ofwhich may be incorporated into the embodiment of FIG. 2, or in otheracoustic temperature measurement systems that do not physically isolatethe waveguide cavity from combustion gas in the adjoining flow path. Thecavity 86 of the waveguide is a tapered cavity which extendssymmetrically along an axis, A, between first and second waveguide ends88, 90. The waveguide wall 92, formed along both sections 100, 102 andabout the cavity 86, has a cylindrically shaped exterior wall surface 92e and has, along an interior surface 92 i, a tapered shape complementaryto that of the tapered cavity 86.

The exemplary first waveguide end 88 is in open communication such thatthe flowing combustion gas may influence state variables in the cavity86. Specifically, there may be an influx of combustion gas into thecavity 86 and a transfer of thermal energy into the waveguide wall 92and cavity (e.g., by conduction or convection) toward the second end 98.The gas turbine engine 10 may comprise a plurality of waveguidesincorporating the features of the waveguide 84, each located at aposition of interest along a wall 40 in the flow path 17. The firstwaveguide end 88 is positioned within a cylindrically shaped mountingsleeve 42 that extends outward from the wall 40 and the second waveguideend 90 extends beyond the mounting sleeve 42. As described for theembodiment of FIG. 2, the mounting sleeve 42 may have an inside diametersubstantially larger than the outside diameter of the exterior wall 92 eto provide a cooling channel, C.

The waveguide 84 comprises first and second connected sections 100 and102. In one embodiment the first section 100 is formed of hightemperature metal, e.g., a stainless steel alloy, and extends between acentral transition region 104 between the two sections 100, 102 to thewaveguide end 88 which adjoins the flow path 17. More generally, thefirst section 100 may be formed of a material composition predominantlycomprising a metal or ceramic material and having a melting pointexceeding 800 C. The waveguide end 88 of the first section 102 isconnected along a wall 40 of a flow path 17 for fluid communication oracoustic communication with the combustion gas.

The second waveguide section 102 is a body formed of an electricallyisolating material having low thermal conductivity properties, e.g.,those of a thermoplastic polymer or a ceramic material, perhaps lessthan 0.4 watts/m-K. The waveguide section 102 may have a volumeresistivity greater than 10¹⁵ ohm-cm, More generally, the second section102 is formed of an electrically isolating material composition having alower thermal conductivity than the material of the first section 100(e.g., by eighty to five hundred times). Numerous ceramics may besuitable for the material of the second section 102 including ceramicscontaining zirconium oxide (ZrO₂) and which may be partially stabilizedwith yttrium. The material of the second section 102 predominantlycomprises nonmetal.

In one series of embodiments the material of the first section has ahigher melting temperature than the material of the second section. Inanother series of embodiments the material of the second section 102 hasgreater electrically isolating properties than the material of the firstsection 100. That is, the material of the second section 102 is moreelectrically insulative than the material of the first section 100. In athird series of embodiments the material of the second section 102 has alower thermal conductivity than the material of the first section 100.In other embodiments, the material of the first section 100 may be aceramic or cermet, the material of the second section 102 may be aceramic or cermet and the sections 100 and 102 may both comprise aceramic or cermet.

The section 102 extends from the central transition region 104 to thesecond waveguide end 90. Per the embodiment shown in FIG. 2, an acoustictransmitter 110 may be attached along the second waveguide end 90. Anacoustic receiver 112 may also be positioned in the second waveguidesection 102, i.e., in an operating environment which benefits from theelectrically isolating and low thermal conductivity properties of thesecond waveguide section 102. This enables the acoustic instrumentationto operate under lower temperature conditions than present along thefirst waveguide section 100.

The material of the first section 100 exhibits strength and stabilitywhile directly exposed to high temperatures present in an adjoining flowpath 17 during turbine operation. The section 102 exhibits low thermalconductivity to help limit thermal conduction between the transitionregion 104 and the waveguide second end 90, and limit the maximumtemperature to which acoustic instrumentation mounted in the secondsection is subjected during turbine operation. Use of an electricallynon-conducting material in the second section 102 of the hybridwaveguide 84 is advantageous when using an acoustic transmitter whichgenerates a high voltage spark. The electrically isolating waveguidesection 102 insulates the receiver electrically and magneticallyisolates the receiver from EMF generated by the spark. Also, groundingconcerns are removed because the sensors are isolated from the engine,thereby avoiding creation of ground loops.

The connected sections 100, 102 of the waveguide 84 may be formed inmultiple ways. In one series of embodiments the waveguide is fabricatedin an additive manufacturing process that incorporates distinct materialcompositions to provide different performance objectives in theadjoining sections.

Portions of the waveguide can be built up in discrete layers, e.g., 10to 30 microns thick, with a graded deposition of blended powders totransition from a first region predominately comprising a first material(e.g., a metal) to a second region predominately comprising, forexample, a thermoplastic or ceramic material that isolates acousticinstrumentation from temperature excursions and damage due to heatconduction. Particles of differing material types may be used whendepositing the various layers, L_(i), with a ratio of the types ofmaterials changing between layers to produce a functional gradient inthe transition region 104. Use of the thermoplastic or ceramic materialto form the second section 102 also prevents electrical and magnetizingeffects prevalent in a waveguide made entirely of metal. This and otherforms of hybrid waveguides can be manufactured with shorter fabricationtimes (e.g., less than half the time) while improving the overallperformance of the acoustic temperature measurement system.

One such process builds a portion of the waveguide 84, e.g., thetransition region 104, with a sequence of discretely formed layers,L_(i), one layer over another layer, which introduce a gradient inmaterial composition. Functionally graded materials are characterized bya gradual change in composition over a volume. Such materials avoid thedisadvantages sometimes associated with abrupt material changes. Ametal-ceramic gradient composition is described in U.S. Pat. No.6,322,897 as being formed by sintering a packed bed of powder having agraded composition across the bed.

In one embodiment the first section 102 is formed of a stainless steelalloy with casting or machining processes. A bond layer of material isapplied directly against the steel alloy material of the first section100. This may comprise an alloy powder thickness of 1-4 mm under a fluxpowder thickness of 2-5 mm. Exemplary layer thickness variants, fluxtypes, laser types and typical power level for processing are describedin US2015/0030871 incorporated herein by reference. After processing alayer of slag is removed prior to application of subsequent layers.After formation of the bond layer graded sequence of deposited layers isapplied to transition the material composition to predominately orentirely ceramic. See FIG. 4 which illustrates a portion of thetransition region 104 in which such layers L_(i) extend along a planeperpendicular to the axis, A. The layers L_(i) may be deposited by athermal spray process, such as High Velocity Oxy-Fuel (HVOF) or AirPlasma Spray (APS). As described in US2015/0030871, the layers L_(i) maybe formed by depositing layers of powder comprising materials whichresult in a desired percent composition. Specifically, the abovedescribed bond layer, referenced as layer L₁, may comprise particles ofthe metal alloy present in section 100 and a flux material. Generally,this first layer L₁ is formed on a surface S of the section 100comprising the alloy (e.g., steel). The surface, S, is alsoperpendicular to the axis, A. The bond layer L₁ may incorporate materialhaving thermal barrier properties (e.g., MCrAlY). Subsequent layersbeginning with the layer L₂ change in proportion of bond layer materialto ceramic material from a relatively higher concentration of bond coatmaterial in layer L₂ to a relatively higher concentration of ceramicmaterial, where the concentration of ceramic material increases witheach subsequent layer L_(i+1) deposited over a prior layer L_(i) untilthe last in the sequence of graded layers (e.g., L₇) comprises 100percent ceramic material or another composition consistent with desiredcomposition of the section 102. With the transition region so formed thesection 102 may also be fabricated with an additive manufacturetechnique (not shown), e.g., continued deposition of materials andprocessing to form a series of layers comprising the desired compositionof the section 102.

FIG. 3 also includes a table showing relative proportions of alloyparticles, bond layer particles and ceramic particles in each respectivelayer associated with the transition region 104. The column titled“Flux” indicates that each layer is deposited using a powder depositionprocess as described in U.S. 2015/0030871, such as laser melting orlaser sintering.

Layers L₁ through L₇ are only exemplary of a graded system of layers totransition the composition of a structure formed by powder deposition ofa plurality of layers of material with the composition of the layersvarying along the axis, A. Different combinations and quantities ofthese types of layers or other types of layers may be included in otherembodiments. For example, in one embodiment the sequence may add orremove layers from those which are illustrated. Multiple steps of moregradually changing composition ratios may be used in other embodiments.Some layers may include additional alloy composition or bond layers.Layers may be of equal or varying thicknesses. Multiple compositions ofalloy material, bond layer materials or ceramic materials may be used ina sequence which forms the transition region 104.

The sections 100 and 102 of the waveguide 84 may also be fabricated asdiscrete components which are subsequently joined together by, forexample, fashioning mating portions 100′ and 102′ with the transitionregion 104 being an interface between the two sections. Joining at theinterface may be had with interlocking patterns formed on the matingportions or with a clamping collar or by encasing the components toestablish a mechanical connection and stability during engine operation.

In other embodiments, the waveguide 84 of FIG. 3 may also be isolatedfrom the hot combustion gas by an acoustically transparent membrane asdescribed in FIG. 2. Moreover, one or more active cooling circuits maybe incorporated to provide active cooling of the entire waveguide 84 oronly portions of the waveguide 84, such as only the first section 100.Other embodiments may include a first section formed of metal, ceramic,ceramic matrix composite, cermet or other material capable of operationunder high temperature conditions, and a second section may be formed ofmaterials such as a ceramic or thermoplastic to provide electrical,magnetic and/or thermal isolative properties. The acousticallytransparent membrane 58 may be located at or near the junction of thefirst and second sections, in lieu of or in addition to locating such amembrane at the first end 32 of the cavity.

There has been described an acoustic apparatus and method for accurate,non-intrusive characterization of hot gas flow temperature, velocity andmass flow. The disclosed system may incorporate multiple transmittersand multiple receiving sensors along the flow path of a combustion gaswhich exits the combustor section of a turbine engine. Placement of theacoustic transmitters and receivers within a waveguide cavity, which issealed to prevent ingress of the hot combustion gas, isolates theacoustic devices from very high operating temperatures which wouldimpose more stringent requirements for operation or more stringentcooling requirements to prevent more frequent instrumentation failures.Furthermore, the apparatus and method enable a more accuratedetermination of temperature information because the acousticinstrumentation is isolated within the waveguide. Thus any cooling fluidbeing circulated to protect the instrumentation does not extend into thecombustion gas flow path. Also, because the chamber 36 is isolated fromthe flow path, cooling methods can incorporate cooler air that has beenused when the instrumentation is directly in the flow path.

In one series of embodiments there have been described a method ofoperating a gas turbine engine which exhausts combustion gas along aflow path and a gas turbine engine incorporating an apparatus forcontrolling turbine operation based on monitoring combustion gasoperating temperature and other parameters such as velocity and massflow rate.

The method provides time of flight information based on propagation ofan acoustic signal to determine an operating temperature of thecombustion gas while the gas passes through a measurement region in theflow path. A waveguide is positioned for isolation from flow of thecombustion gas while injecting the acoustic signal into a measurementregion. Temperature within the waveguide is limited, relative to thecombustion gas operating temperature, by positioning an acousticallytransmissive physical barrier between the waveguide and the measurementregion to prevent movement of the combustion gas from the measurementregion into the waveguide while permitting transmission of the acousticsignal from the waveguide into the measurement region to acquire thetime of flight information.

In one embodiment at least one device is positioned for transmitting theacoustic signal from the one device into the measurement region, anddetecting return of the acoustic signal into the waveguide after thesignal is reflected from a surface in the measurement region. Time offlight information is determined for the transmission and return of theacoustic signal, and the operating temperature is determined based onthe time of flight information. Generally, the waveguide may comprisecavity adjoining the flow path along which instrumentation is placed totransmit the acoustic signal into the measurement region and monitorreturn of the acoustic signal after reflection from the measurementregion. In one embodiment positioning the acoustically transmissivephysical barrier may include positioning a membrane along an end of thewaveguide to prevent flow of combustion gas into the waveguide. Thewaveguide seals the cavity from receiving combustion gas directly fromthe flow path. The temperature in the waveguide may be limited byinjecting a gas into the waveguide to flow between the first and secondwaveguide ends.

There has been described an acoustic temperature measurement system forproviding acoustic time of flight information for combustion gas in agas turbine engine. Measurement is based on propagation of an acousticsignal to determine an operating temperature of the combustion gas whilethe gas passes through a measurement region in the flow path. Awaveguide has first and second ends and a cavity extending between thefirst and second ends of the waveguide. The first end is positioned toadjoin the flow path for injection of the acoustic signal into themeasurement region. An acoustically transmissive physical barrier ispositioned between a portion of the waveguide and the measurement regionto prevent movement of combustion gas from the measurement region intothe waveguide while permitting transmission of the acoustic signal fromthe waveguide into the measurement region. The physical barrier ispositioned to limit temperature within the waveguide, relative tocombustion gas operating temperature in the flow path.

There has been described a gas turbine engine including an apparatus forcontrolling operation based on monitoring combustion gas operatingtemperature in a measurement region along a flow path. Measurement isbased on determining time of flight information for propagation of anacoustic signal through the measurement region. The engine includes acombustor section and a turbine section along which a flow path extendsfrom the combustor section and a waveguide having first and secondopposing ends. The first end adjoins the flow path to enable injectionof the acoustic signal into the flow path while the waveguide remainsisolated from flow of the combustion gas therein. The waveguide includesa barrier layer across the first end. Acoustic instrumentation,providing the functions of an acoustic transmitter and a receiver, ispositioned along an interior wall of the waveguide to generate theacoustic signal and sense receipt of the acoustic signal afterreflection from the measurement region. The barrier layer may include amembrane comprising titanium or a nickel alloy.

There has also been described a waveguide for use in a gas turbineengine for determining time of flight information for propagation of anacoustic signal through a measurement region in a flow path carryingengine combustion gas. The waveguide comprises a body having first andsecond ends and a tapered cavity extending between the first and secondends, with the first end including a barrier layer so that when thefirst end is positioned to adjoin the flow path to inject the acousticsignal into the flow path, the cavity remains isolated from flow of thecombustion gas along the flow path. The waveguide may also includeacoustic instrumentation positioned along an interior wall of thewaveguide to provide for generation of the acoustic signal andpropagation of the acoustic signal into the measurement region. Thewaveguide may also include cooling passages along a wall thereof tocontrol temperature of the acoustic instrumentation. The coolingpassages may include an inlet port and an outlet port positioned to passthe cooling fluid through a portion of the chamber. The cooling passagesmay include an inlet port and an outlet port each positioned tocirculate cooling fluid through a channel formed along an exteriorsurface of the waveguide.

A hybrid waveguide has also been described. In one series of embodimentsa hybrid waveguide is provided for placement adjacent a flow path forhot combustion gases in a gas turbine engine. The waveguide is a bodymember with first and second opposing ends. The body member includes afirst section formed of a first material extending from a transitionregion to the first end and a second section formed of a second materialextending from the transition region to the second end. The firstmaterial predominantly comprises a metal. The second material comprisesan electrically isolating material having a lower thermal conductivity(e.g., fifty to one thousand times lower) than the material of the firstsection. In one example embodiment the first material predominantlycomprises a steel alloy.

The second material may predominantly or entirely comprise athermoplastic material or a ceramic formulation. The transition regionmay comprise a graded sequence of layers that transition the compositionof the waveguide from the first material in the first section to thesecond material in the second section. The first material in the firstsection may include or be entirely a composition which predominantlycomprises metal and the second material in the second section include orentirely be a composition having a lower thermal conductivity than thatof the first material composition.

While various embodiments of the present invention have been shown anddescribed herein, it will be obvious that such embodiments are providedby way of example only. Numerous variations, changes and substitutionsmay be made without departing from the invention herein. Accordingly, itis intended that the invention be limited only by the spirit and scopeof the claims which now follow.

The claimed invention is:
 1. A measurement system for combustion gas ina gas turbine engine based on propagation of an acoustic signal whilethe gas passes through a measurement region in the engine, the systemcomprising: a waveguide having first and second ends and a cavityextending between the first and second ends of the waveguide, the firstend configured to adjoin a flow path for injection of the acousticsignal into the measurement region; an acoustically transmissivephysical barrier positioned between a portion of the waveguide and themeasurement region to prevent movement of the combustion gas from themeasurement region into the waveguide while permitting transmission ofthe acoustic signal from the waveguide into the measurement region; anda cooling arrangement configured to control an operating temperature ofthe waveguide wherein the cooling arrangement further comprises an inletport and an outlet port extending through a wall defining the cavity todirect cooling fluid through the cavity, and wherein the inlet port isconfigured to direct at least a portion of the cooling fluid onto theacoustically transmissive barrier.
 2. The system of claim 1 wherein theacoustically transmissive physical barrier is a membrane positioned toseal the cavity, the system including at least one device fortransmitting or receiving the acoustic signal along a wall of thewaveguide.
 3. The system of claim 2 wherein the at least one device is atransmitter positioned to generate the acoustic signal for propagationinto the measurement region.
 4. The system of claim 2 wherein the atleast one device is a transmitter positioned to generate the acousticsignal for propagation into the measurement region, the system furtherincluding a receiver positioned along a wall of the waveguide to monitorreturn of the acoustic signal after reflection from the measurementregion.
 5. The system of claim 1 wherein the inlet port is positionedcloser to the waveguide first end than to the waveguide second end andthe outlet port is positioned between the inlet port and the waveguidesecond end, causing the cooling fluid to travel in a direction away fromthe first end and toward the second end.
 6. The system of claim 1further comprising a wall surrounding the cavity and wherein the coolingarrangement comprises a channel formed along the wall to direct coolingfluid for controlling the operating temperature of the waveguide.
 7. Thesystem of claim 6, further comprising a series of ports extending fromthe channel through the wall to the cavity to provide circulation ofcooling fluid through the cavity.
 8. The system of claim 1 furthercomprising a wall surrounding the cavity, the wall comprising a firstsection formed of a first material extending from a transition region tothe first end and a second section formed of a second material extendingfrom the transition region to the second end, the first material havinga higher melting temperature than the second material of the secondsection.
 9. The system of claim 1 further comprising a wall surroundingthe cavity, the wall comprising a first section formed of a firstmaterial extending from a transition region to the first end and asecond section formed of a second material extending from the transitionregion to the second end, the second material having greaterelectrically isolating properties than the first material.
 10. Awaveguide for use in a gas turbine engine for determining time of flightinformation for propagation of an acoustic signal through a measurementregion in a flow path carrying engine combustion gas, the waveguidecomprising: a body comprising first and second ends and a tapered cavityextending between the first and second ends, with the first endcomprising an acoustically transmissive physical barrier layer so thatwhen the first end is positioned to adjoin the flow path to inject theacoustic signal into the flow path, the cavity remains isolated fromflow of the combustion gas along the flow path; and acousticinstrumentation positioned to provide for generation of the acousticsignal and propagation of the acoustic signal into the measurementregion, a cooling passage configured to control temperature of theacoustic instrumentation during operation of the gas turbine engine,wherein the cooling passage comprises an inlet port and an outlet portpositioned to pass a cooling fluid through a portion of the cavitywherein the inlet port is configured to direct at least a portion of thecooling fluid onto the acoustically transmissive barrier layer.
 11. Thesystem of claim 10 wherein the cooling passage comprises a channelformed in the body.